Four series of studies are proposed to investigate the hypothesis that age-related impairments in reasoning and spatial abilities are attributable to a decrease with age in the rate of processing information. The first phase of the project is designed to examine the complexity effect phenomenon (i.e., the trend for the magnitude of the age differences to increase with the complexity of the task) in reasoning and spatial domains. Experiments in the second phase will determine whether, and if so by how much, older adults differ from young adults in the duration of component processes of prototypical reasoning and spatial ability tasks. The goal of the Phase 3 studies is to provide an empirical demonstration that variations in rate of processing can result in substantial differences in quality of performance. The final phase in the project is to construct a working computer simulation of two prototypical tasks to explore the possibility of reproducing the age differences in performance by manipulating only rate parameters in the simulation. The goal is to explore the feasibility of the proposal that reductions in the efficency of cognitive functioning associated with increased age are attributable to a slower rate of processing information Result from the proposed studies should not only contribute to greater understanding of reasons for adult age differences in cognition, but may also be useful in attempting to discover mechanisms by which health variables influence mental functioning.